This is one of a suite of operations on finite partial functions, type
('a,'b)func. These may sometimes be preferable to ordinary functions since
they permit more operations such as equality comparison, extraction of domain
etc. The graph function takes a finite partial function that maps x1 to
y1, ..., xn to yn and returns its graph as a set/list:
[x1,y1; ...; xn,yn].

FAILURE CONDITIONS

Attempts to sort the resulting list, so may fail if the type of the pairs does
not permit comparison.